The present invention relates to an improved method and apparatus for removing dissolved solids from a liquid. In particular the invention is directed to an improved method and apparatus for treating a liquid which utilizes the technique of reverse osmosis. The term reverse osmosis is referred to hereinbelow and in the claims as "RO".
In its most basic presentation RO is a membrane process that acts as a molecular filter to remove up to 95-99% of all dissolved minerals, 95-97% of most dissolved organics, and more than 98% of biological and colloidal matter from water. RO can be described as the separation of water from dissolved solids by application of a pressure differential across a membrane which is permeable to water, but not to the dissolved solids. The applied pressure differential must be sufficient to overcome the forces tending to keep the water and dissolved species together. In actual practice, the semi-permeable membrane is contained in a pressure vessel which is compartmentalized by the presence of the membrane. A concentrated solution is introduced under pressure to one side of the membrane and the solvent (water) passes through the membrane. As the solvent is separated from the solution, the solution becomes more and more concentrated. In order to maintain a continuous process, the highly concentrated solution must be drawn off.
The most basic problem in designing an RO device, or a piece of apparatus to use the semi-permeable membrane, is how to package a thin membrane. Flow of water through the membrane is directly proportional to the area of the membrane and inversely proportional to the thickness. Therefore, it is obviously desirable to find some means of packaging the greatest possible area of the thinnest possible membrane. This consideration, and numerous others of varying degrees of importance, have led to the design of three types of RO devices in current use today; the tubular device, the spiral wound device, and the hollow fine fiber device.
The tubular device comprises a semi-permeable membrane which is either inserted into, or coated onto, the surface of a porous tube, which is designed to withstand the operating pressure. Feed water under pressure is introduced into the end of the tube, and the product water permeates through the membrane and the tube, and is collected on the outside. The reject or concentrate exits from the far end of the tube. Although this type of RO device achieved some degree of commercial success, especially in non-water applications such as chemical separations and food and drug processing, the cost of such systems is prohibitively expensive for water treating because of the small membrane area per unit volume of container.
The spiral wound device comprises a jelly roll like arrangement of feed transport material, permeable transport material, and membrane material. At the center of the roll is a perforated permeate collector tube. Several rolls are usually placed end to end in a long pressure vessel. Feed water enters one end of the pressure vessel and travels longitudinally down the length of the vessel in the feed transport layer. Direct entry into the permeate transport layer is precluded by sealing this layer at each end of the roll. As the water travels in a longitudinal direction, some of it passes in the radial direction through the membrane into the permeate transport layer. Once in the transport layer, the purified water flows spirally into the center collection tube and exits the vessel at either end. The concentrated feed continues along the feed transport material and exits the vessel on the opposite end from which it entered.
The hollow fine fiber device comprises a bundle of porous hollow fine fibers. These fibers are externally coated with the actual membrane and form the support structure for it. The hollow fibers are assembled in a bundle lengthwise within a pressure vessel. Feed water under pressure enters the pressure vessel through a highly porous distributor located at the center of the hollow fiber bundle. The distributor runs the entire length of the pressure vessel. Water moves radially outward from the distributor towards the outer shell of the vessel, still essentially at feed pressure. The pressure forces the pure water through the fiber walls into the hollow center of the fiber, and this water moves along the hollow center of each fiber to a tube sheet end, where the fibers have been set in epoxy and cut to allow the pure water to exit. The salts, minerals, and other contaminants remaining in the water move to the outer perimeter of the vessel and are taken out of the vessel through a reject brine port. The fibers at this end are sealed. A hollow fiber membrane allows one to pack the maximum amount of membrane surface area into a given vessel. Accordingly, such devices are more compact than the other devices.
It is the primary function of RO devices to remove dissolved species, and the heretofore designs of RO modules have been optimized to do so in the absence of appreciable quantities of larger suspended material. These larger particles tend to clog the filter surface or the waste stream channels and thereby foul the RO module. It has therefore been the heretofore practice to use conventional filtration and polishing pretreatment equipment to remove suspended matter from the feed water prior to entry into the RO system. The specific pretreatment requirements are determined dependent upon the particular characteristics of the feed water.
It has long been recognized that hollow fiber RO elements are easily fouled due to particulate matter in the feedstream and the precipitation of salts in areas of low flow velocity. This is primarily due to poor flow distribution and the filtering action of the fiber bundle. In current hollow fiber RO elements the feedstream is introduced through a central perforated tube which permits the feedstream to migrate radially outward to the pressure vessel circumference. During this migration, the velocity of the feedstream is reduced due to the removal of permeate and the increasing diameter of the fiber bundle. Current hollow fiber RO devices make no attempt to prefilter the feedstream within the RO pressure vessel. Further, once the current elements have become fouled in the aforementioned manner, the lack of good flow distribution in any direction materially hinders any form of efficient cleaning of the fiber bundle. As hereinabove mentioned, all attempts to remedy this situation have centered on external filtering techniques to reduce the concentration of particulate matter in the entering feedstream.